Don't use openblas cmake builds

[Sven Baars]

Please don't use openblas cmake builds. They are still experimental as is stated during the build process:

CMake Warning at CMakeLists.txt:46 (message):
  CMake support is experimental. This will not produce the same Makefiles
  that OpenBLAS ships with. Only x86 support is currently available.

There are still bugs with the cmake build. I just found one that causes DTRMM to give wrong answers:

https://github.com/xianyi/OpenBLAS/issues/951

It took me quite a long time to figure out that this was a bug in blas and not in my own code. I request that you stick with standard builds without cmake until the cmake support is not experimental any more. So people can at least use openblas without having to worry about really odd bugs.

[Julian Taylor (jtaylor)]

what are you referring too?
the openblas package does not use cmake

[Sven Baars (sbte)]

Huh, that's interesting. I encountered this bug while using the system provided version of openblas on Ubuntu 16.04. And I just fixed the bug in openblas itself by adjusting the cmake build files, so I assumed that that was where the bug came from in the openblas version that was provided by Ubuntu as well. I guess not. So there must be another bug in openblas then...

I guess I'll have to investigate further. Maybe there's a bug in the standard implementation of trmm. Anyway, sorry for the noise.

[Julian Taylor (jtaylor)]

the cmake change in the github issue does not fix the problem, it just causes you to not use the broken code anymore.
can you provide a compile and runnable example for me to try?

[Julian Taylor (jtaylor)]

if I had to guess the reason why the package does not work is that the version in 16.04 does not detect skylake cpus and falls back to some other cpu.

[Sven Baars (sbte)]

I'll just try to fix the bug itself tomorrow. It should be much easier to trace it down now that I know that I probably need to compile it with the default build target to be able to bisect it (since it worked fine in Ubuntu 14.04). I'll let you know when I find the problem.

[Julian Taylor (jtaylor)]

you used the same skylake cpu on the ubuntu 14.04 version?
its probably a good bet to assume the issue was always there instead of it being a regression. openblas is notorious for giving wrong results depending on the hardware used.